There are many different etch solutions disclosed in the prior art for etching copper particularly for producing electronic circuits from copper laminates using etch resistant materials such as tin-lead (solder) resists. Etching solutions in use today include ammoniacal copper etchants, chloride copper etchants chromic acid etchants, ferric chloride etchants and hydrogen peroxide-sulfuric acid etchants.
Regardless of the particular etching solution employed, users always have the problem of what to do with the spent etching solution once it has become saturated with copper. Spent etchants contain large amounts of copper metal and the supplier of the etchants generally takes back the spent etchant and gives the user some credit for this spent solution. Hydrogen peroxide-sulfuric acid etchants, however, can carry only about half or even less than half of the copper carried by the ammoniacal or copper chloride etchants before they are spent or no longer usable. The users of hydrogen peroxide-sulfuric acid etchants generally do not return the copper-laden spent etchants to the supplier, but instead pump the spent solution out of the etching chamber and cool the solution sufficiently to permit crystallization of copper sulfate crystals. After the crystallization at reduced temperature, the solution, which is now fairly low in copper content, is then pumped back into the etching chamber for formula readjustment and reuse. One of the advantages of the hydrogen peroxide-sulfuric acid etchant is the ability to reuse the etching solution in the users facility with minimal expense in equipment, whereas with other etchants the solution is sent back to the supplier or discarded. The main disadvantage in reusing the hydrogen peroxide-sulfuric acid solutions is that the user is faced with a voluminous amount of copper sulfate crystals which must be handled, sold, or disposed of. These crystals are sharp and produce significant wear on the pumps that are being used to handle the crystals, and this, together with the necessity of handling large amounts of the crystals and finding a buyer or acceptable disposal site, makes this recovery process a difficult one for the user.
It has also been proposed to remove copper metal from the hydrogen peroxide-sulfuric acid etchants by plating out the copper electrolytically. The problem with this procedure has been that the copper metal does not deposit as an adherent homogeneous deposit but comes out as a powder or in a non-adherent loose formation which powders off into the solution and this makes recovery and handling of the copper very difficult. The use of certain grain refiners has also been proposed; however, these are not stable in the presence of a strong oxidizing agent under the conditions of electrolysis.
There are other methods that separate copper sulfate by membrane technology or by dissolving the copper sulfate crystals and then electrolyzing the solution but this involves additional steps and apparatus and is much more complicated than the method of the present invention.
U.S. Pat. No. 3,903,244 to Winkley discloses the stabilization of hydrogen peroxide solutions with amino methyl phosphonic acid or its soluble salts, and when these solutions are used in metal pickling the phosphonic acids precipitate iron contained in the solutions from the pickling operation and thus stabilizes the hydrogen peroxide against decomposition induced by the presence of ferric iron ions. Phenol is disclosed as useful in the compositions to control decomposition of hydrogen peroxide induced by non-ferric heavy metals, such as copper.
U.S. Pat. No. 3,905,907 to Shiga discloses hydrogen peroxide solutions incorporating an acid together with an alkyl hydrogen phosphate or an alkyl hydrogen phosphite. Phosphoric acid is mentioned as a possible etching acid along with sulfuric and other acids.
U.S. Pat. No. 3,373,113 to Achenbach discloses hydrogen peroxide sulfuric acid etching solutions containing phosphoric acid as a stabilizer for the hydrogen peroxide. The solutions are used to etch printed circuits in which the circuit is defined by an etch-resistant printing ink.